The present invention concerns the impregnation of film-foil capacitors. More particularly, it concerns a gel impreggnant that adheres well to film and foil materials and that is flexible enough to expand and contract during thermal cycling.
Electrical capacitors are used for storing energy in a variety of applications. Operating voltages for such capacitors range from a few volts (e.g. miniature or micro electronic circuitry) to thousands of volts (e.g. power utility applications). A capacitor comprises a pair of conductive plates or electrodes separated by a dielectric material. The electrodes are typically composed of copper, silver, aluminum foils or vacuum deposited zinc or aluminum. Capacitors utilize a variety of dielectric materials ranging from ceramics, metal oxides, plastic sheets or films to paper.
Higher voltage capacitors are generally constructed of multiple sheets of a dielectric material such as polypropylene or polyester film in between sheets of foil such as aluminum foil. These materials are typically wound into a roll and vacuum impregnated in roll form or collapsed into rectangular elements and then vacuum impregnated. Capacitors intended to operate at voltages greater than 600 V are normally completely impregnated with a low viscosity dielectric-liquid with good gas absorbing properties.
It is important for the operation of high voltage capacitors that all void spaces be filled. Otherwise there will be corona breakdown of voids, which will lead to low breakdown strength and failure. Thus, it is known to impregnate such capacitors with a liquid impregnant such as mineral oil, castor oil, polybutylene, dioctyl phthalate and other liquid impregnants.
It is also known to impregnate such capacitors with epoxy and urethane solid dielectric materials. The impregnants generally fill the void spaces in the capacitor to increase capacitance, reduce corona discharges and aid in the transfer of heat from the capacitor to the outside environment.
On the one hand, it is desirable to use an impregnant that is not liquid so that there will be no leaking of fluid, with its possible negative environmental impact, in the event of a capacitor failure or in case the capacitor case is not well sealed. However, voids can be created in a solid impregnant when it cures. Moreover, voids can develop more readily with a solid impregnant during thermal cycling of a capacitor in service, where temperatures can range between xe2x88x9255xc2x0 C. and 85xc2x0 C. In particular, low temperatures can cause solid capacitor materials to contract, which can open up voids in the capacitor. If these voids are adjacent to a foil edge where there is a high electric stress, there can be a gas void breakdown at high voltages that would cause the capacitor to fail at unusually low breakdown voltages. Thus, it is not common to use solid or gel materials as impregnating materials for high voltage capacitors.
Rather, despite environmental concerns, high voltage capacitors are commonly impregnated with a low viscosity liquid with good gas absorption properties. The liquid is better able to expand and contract along with the dissimilar capacitor materials during thermal cycling to avoid formation of voids within the capacitor. Yet, it would be desirable to have a gel impregnant that is flexible enough and adherent enough to the capacitor materials to prevent voids from occurring. It would also be desirable to have a dielectric gel impregnant with good insulating properties, high dielectric constant and good compatability with capacitor materials.
The normal impregnation process begins by placement of capacitors to be impregnated in a chamber that is then evacuated. The vacuum chamber is flood filled with the impregnant so that the capacitors inside the chamber are covered. Thereafter, the chamber is exposed to atmospheric pressure, which aids in forcing the impregnant into any voids in the capacitor. While this method is satisfactory for low viscosity liquid impregnants, it does not always work well with higher viscosity materials or gels. Accordingly, an improved, more reliable process for impregnation would be desirable.
According to the present invention, a gel impregnant has been developed that adheres well to film and foil materials of a film-foil capacitor and is flexible enough to expand and contract during thermal cycling without creating voids. Film-foil capacitors made using the gel impregnant have increased capacitance and energy density, and have high breakdown voltages and a breakdown voltage distribution that has very low two sigma limits (less than 10% of the average breakdown voltage) Moreover, the impregnant is a gel rather than a liquid, thereby reducing the possibility of leakage from the capacitor.
The preferred composition of an impregnant precursor according to the present invention can comprise up to five components: (1) a plasticizer; (2) a primary polyol; (3) a maleic anhydride able to react with the primary polyol; (4) a crosslinking material to impart toughness to the finished gel; and (5) a catalyst. The fourth component is optional but presently preferred to improve the toughness of the cured gel. At least a small amount of a catalyst is needed to aid the curing process.
Thus, the preferred elastomeric gels once cured are polyesters formed by the reaction of a polyol with a maleic anhydride. The polyester is blended with an unreactive component (i.e. a plasticizer) which provides the softening point behavior and imparts the elastomeric properties of the bulk dielectric impregnating material. It also lowers the viscosity of the blend, which facilitates the impregnation process. The plasticizer is preferably an economical and environmentally safe material that does not react with the film and foil of the capacitor.
According to the principles of the invention, film-foil capacitor rolls are constructed. The liquid components that comprise the impregnating gel are mixed and de-gassed under vacuum prior to impregnation. The capacitor rolls are vacuum dried and the chamber containing the capacitor rolls are back-filled with de-gassed liquid components that comprise the impregnating gel. At this stage the liquid components preferably are in the form of a medium viscosity reactive liquid.
After being submerged in the liquid mixture, the capacitor rolls are pressurized with the liquid mixture in order to insure complete impregnation. Preferably, the pressure applied is in excess of 100 psi. The pressure on the capacitor rolls is maintained while the liquid components are cured to form the impregnating gel. The gels are cured at room temperature or higher over a period of time ranging from a few minutes to a few hours depending on the temperature used and the formulation of the gel components. The curing time can be reduced by elevating the cure temperature or by increasing the concentration of either or both of the reactive components or any catalyst component. Preferably, a temperature above 25xc2x0 C. is used for curing. The impregnated capacitors are then recovered from the curing chamber, allowed to cool if necessary and subsequently encased and/or sealed in a case or container.
Capacitors constructed from sheet materials and gel impregnating dielectric materials according to the principles of this invention have unusually high breakdown voltage and even after thermal cycling retain a very high breakdown voltage. Also, the statistical distribution of the breakdown voltage is unusually tight. The dielectric gel impregnating materials of the present invention comprise solid impregnating materials formed by curing liquid reactants with low shrinkage during curing and reduced gas voids upon curing. Moreover, the elastomeric, dielectric impregnating gels exhibit good adhesion to the metallized film and foil capacitive materials and are flexible enough to expand and contract during thermal cycling without creating gas voids.
A gel impregnated capacitor of the present invention has improved properties of capacitance, increased energy densities and breakdown voltages, as compared to preexisting capacitors. It is believed, that these advantages are achieved both because of the high pressure impregnation, particularly the use of high pressure during curing of the impregnant, and because of the stickiness and softness of the resulting gel material. The gel has a high adhesive property to the film and the foil and does not come loose even with the most rigorous thermal cycling. Also, the softness or flexibility of the gel material allows it to follow the expansion and contraction of the other capacitor materials during thermal cycling.